In the recovery of oil from subterranean, oil-bearing formations or reservoirs, it is usually possible to recover only a limited proportion of the original oil present in the reservoir by the so-called primary recovery methods which utilize the natural formation pressure to produce the oil through suitable production wells. For this reason, a variety of supplementary recovery techniques have been employed, directed either to maintaining formation pressure or to improving the displacement of the oil from the porous rock matrix. Techniques of this kind have included formation pressurization, thermal recovery methods such as steam flooding and in situ combustion, water flooding and miscible and immiscible flooding techniques.
In miscible flooding operations, a solvent is injected into the reservoir to form a single phase solution with the oil in place so that the oil can then be removed as a more highly mobile phase from the reservoir. This provides extremely effective displacement of the oil in the areas through which the solvent flows, so that an extremely low residual saturation is obtained. The efficiency of this process derives from the fact that under the conditions of temperature and pressure prevailing in the reservoir, a two-phase system within the reservoir between the solvent and the reservoir oil is eliminated. When this happens the retentive forces of capillarity and interfacial tension which are significant factors in reducing the recovery efficiency of oil in conventional flooding operations where the displacing agent and the reservoir oil exist as two separate phases, are eliminated or substantially reduced.
Miscible recovery operations are normally carried out by a displacement procedure in which the solvent is injected into the reservoir through an injection well to displace the oil from the reservoir towards a production well from which the oil is produced. Because the solvent, typically a light hydrocarbon such as liquid petroleum gas (LPG) or a paraffin in the C.sub.2 to C.sub.6 range, may be quite expensive, it is often desirable to carry out the recovery by injecting a slug of the solvent, followed by a cheaper displacement liquid such as water. However, the economics of miscible recovery operations using first contact miscible solvents such as LPG or light hydrocarbons are quite unattractive.
Of the various recovery processes so far used or proposed, flooding by carbon dioxide is considered to be of substantial promise. In the carbon dioxide flooding technique, a slug of carbon dioxide is injected into the formation to mobilize the oil and permit it to be displaced towards a production well. Even under conditions where the carbon dioxide is not wholly effective as a solvent for the oil, recovery may be improved by taking advantage of the solubility of carbon dioxide in the oil, causing a viscosity reduction and a swelling of the oil, which leads to increased recovery. These effects have been utilized at pressures much lower than the miscibility pressures for carbon dioxide and oil. Processes using carbon dioxide as a recovery agent are described in U.S. Pat. Nos. 3,811,501, 3,811,502, 3,497,007, 4,299,286 and 4,410,043.
Carbon dioxide is not a first contact miscible solvent like LPG or a light hydrocarbon, which forms a single phase solution with the reservoir when the two come into contact, i.e. upon their first contact. Rather, carbon dioxide is capable of forming a single phase under appropriate conditions but only after a period of time during which the carbon dioxide first preferentially extracts the light hydrocarbons containing from two to six carbon atoms from the crude oil, thereby developing a hydrocarbon-containing solution at the interface between the carbon dioxide and the crude oil. This solution is able to dissolve other, heavier hydrocarbons, i.e. C.sub.6+ hydrocarbons and these progressively enter the solution to form a single phase which is then carried forward through the reservoir, progressively dissolving heavier hydrocarbons as it advances. Thus, as the flooding front advances through the reservoir, the composition of the displaced fluid gradually changes from the crude oil to that of the pure carbon dioxide under these conditions.
Multiple contact miscibility is a function of the pressure of the system and the minimum pressure required to achieve multiple contact miscibility is called the minimum miscibility pressure or MMP. This varies according to the nature of the oil and of the solvent and in accordance with certain other factors. In some reservoirs, the minimum miscibility pressure may be unattainable due to factors such as low overburden pressure or the impracticality of pressurizing the reservoir. Also, the MMP is usually very high for low gravity viscous oils. If the minimum miscibility pressure cannot be achieved in the reservoir, the flooding process will be immiscible in character and recovery from the solvent injection will generally tend to be low.
In application Ser. No. 704,232, I described a method for improving the recovery of reservoir oil under conditions which normally would have been unfavorable for miscible flooding. In the method described in that application, a solubility-improving additive such as butane is injected with the initial slug of solvent to lower the minimum miscibility pressure (MMP) to a value which is no higher than the prevailing reservoir pressure. In this way, miscibility may be attained even in reservoirs at a relatively low pressure. The additive is progressively removed from the reservoir as the flooding operation continues by injecting subsequent slugs of solvent with reduced amounts of the additive although the operation continues to proceed under miscible conditions as the solubility-improving additive which remains in the formation behind the advancing solvent front is picked up by successive solvent slugs containing less than the equilibrium amount of the additive.
Although that process is capable of improving recovery under otherwise unfavorable conditions of low reservoir pressure it does have certain limitations. One, of course, is that if the reservoir pressure is too low it may not be possible to reduce the MMP of the solvent/oil system to that level, except possibly by the use of wholly uneconomic amounts of the solubility additive. Thus, the method described in my earlier application may not be susceptible of universal application, especially in heavy oil reservoirs or in low pressure reservoirs where re-pressurization is either not possible or economically unattractive. This implies that it may, in such cases, be necessary to carry out the flooding under immiscible conditions where the flooding liquid forms a two-phase system with the reservoir oil. As mentioned above, this is generally expected to reduce the efficiency of the recovery to a level below that of a miscible operation. There is a need, therefore, to improve the recovery under these circumstances.